If you have issues with packages unable to locate libstdc++.la,
then try running 'fix_libtool_files.sh' on the old gcc versions.
You might want to review the GCC upgrade guide when moving between
major versions (like 4.2 to 4.3):
http://www.gentoo.org/doc/en/gcc-upgrading.xml

An ABI, or Application Binary Interface, is a set of conventions used by all tools that deal with binary representation of programs, including compilers, assemblers, linkers and language runtime support (source: GCC Binary Compatibility). When the ABI used for binary applications and libraries is changed, you will risk getting linker errors or malfunctioning programs unless you rebuild all libraries that use C++ code. Yes, C++, since most incompatibilities occur within the C++ ABI. This is also why we use the revdep-rebuild command against the libstdc++.so.5 library.

So why is this only needed up to GCC 3.4.0/4.1 ? That's because from that version onwards, GCC uses a forward compatible ABI, which removes the need for rebuilding applications and libraries. Of course, guarantees can never be given indefinitely, but when an incompatibility occurs again, we'll definitely document it here In that case, the version of the libstdc++.so library will probably be increased.

Rebuilding Everything

Some people swear that they need to rebuild every single package on their system when a new GCC version is made available. Of course, that doesn't make sense, since there are many applications that are not using GCC for their build and install process anyhow, so they would never be affected by such changes.

That however doesn't mean they are completely incorrect: newer GCC versions often include better support for the processors' instruction set, which might influence the performance of some applications in a positive way. Although it is expected that this improvement is generally only marginally, in some cases (especially CPU intensive applications) this might yield notable improvements.

There are also known cases where packages need to be built with the same compiler. Although these packages are usually bumped by Gentoo simultaneously (so that they are always built with the same GCC version) cherry-picking reinstalls on these packages might prove to be troublesome. The various qt-* packages are a nice example on this matter.